Roughing end mills equipped with replaceable inserts have been proposed, for example, in Patent documents 1 and 2. In these roughing end mills, a plurality of twisted grooves are formed in an outer circumference of a circular column-shaped tool body, and square plate-shaped inserts are fitted inside these twisted grooves with a slight gap between each insert. The inserts are positioned substantially equidistantly in the twisted grooves such that the inserts of each twisted groove cover the gaps between the inserts in the other twisted grooves. Here, four twisted grooves are formed in the roughing end mill described in Patent document 1, with the first and third twisted grooves being the same as each other and the second and fourth twisted grooves being the same as each other. In addition, first inserts which substantially overlap a rotation outer edge of the first inserts in the first and third twisted grooves are placed at a distal end side of second inserts of the second and fourth twisted grooves.
Moreover, in the roughing end mill proposed in Patent document 2, three twisted grooves are formed therein, with the second and third twisted grooves being the same as each other. In addition, first inserts which substantially overlap at a rotation outer edge of the first inserts in the first twisted groove are located at a distal end side of second inserts of the second and third twisted grooves. According to the roughing end mills of Patent documents 1 and 2, by suppressing tool damage by making the distal end cutting edges twice as large as the outer circumferential cutting edges, the outer circumferential cutting edges are divided, the cut off chips are fragmented and the cutting force becomes dispersed. As a result, vibration is suppressed and superior cutting is achieved.
PATENT DOCUMENT 1: Japanese Patent Application, First Publication No. H08-1426
PATENT DOCUMENT 2: Japanese Patent Application, First Publication No. H08-1427